1. Field of the Invention
The present invention relates to a process for producing acrylonitrilic separation membranes. More specifically, the invention is concerned with a process for producing an acrylonitrilic separation membrane which comprises shaping a membranous product of a desired form from a solution of a polymer of acrylonitrile (hereinafter abbreviated as AN) dissolved in formamide-containing dimethylformamide (dimethylformamide hereinafter abbreviated as DMF) while maintaining the temperature of said polymer solution within a specified range and removing the solvent from the membranous product or performing the solvent removal under a specific condition, whereby an AN separation membrane (AN filtration material) which is asymmetric, porous in structure and excellent both in separation ability and in mechanical strength is produced.
2. Description of the Prior Art
In recent years, for a wide variety of applications including waste water treatment, desalination of sea water, food industry, medical treatment, etc., separation methods for various substances using semi-permeable membranes have been given attention and have been subjected to further development.
Among such separation methods by means of semi-permeable membranes, there have been proposed various techniques such as ultrafiltration, reverse osmosis, etc., depending upon the particle size of the substance which is suspended, dispersed or dissolved in an aqueous medium. It goes without saying that the properties required for the membrane should be different depending on the methods of separation but among the properties commonly required there can be mentioned a high permeation rate of the aqueous medium, excellent rejection ability, a sufficient mechanical and chemical strength, etc.
A great number of semi-permeable membranes are known at present, but it is not an exaggeration to say that there is no membrane at present which satisfies the above-mentioned properties to a practical level. For example, even in the case of cellulose acetate membranes which are regarded as being relatively practical, defects are observed particularly in resistance to chemicals and resistance to microbic decomposition. On the other hand, because AN polymer membranes are originally excellent in chemical properties, various methods of membrane formation from AN polymers have been proposed. These methods can be known from Japanese KOKAI (Laid-Open) Patent Application No. 53258/1974 (a method wherein the coagulating agent has different coagulation values at both sides of the membrane); Japanese KOKAI Patent Application No. 62380/1974 (a sheath-core type composite extrusion method); Japanese KOKAI Patent Application No. 92359/1975 (a method wherein a mixed solvent is used); Japanese KOKAI Patent Application No. 117682/1975 (a method wherein a sol membrane is formed, followed by solvent evaporation and desolvation), etc. These methods involve disadvantages such as low productivity or membrane thickening (thickening of the membrane layer which has separation activity) and therefore they are not useful from an industrial viewpoint.